Diseases, disorders and conditions where 5-HT2C modulation is desired are for example depression, anxiety, schizophrenia, bipolar disorder, obsessive compulsive disorder, migraine, pain, epilepsy, substance abuse, eating disorders, obesity, diabetes, erectile dysfunction and others.
Serotonin (5-hydroxytryptamine, 5-HT), a monoamine neurotransmitter and local hormone, is formed by the hydroxylation and decarboxylation of tryptophan. The greatest concentration is found in the enterochromaffin cells of the gastrointestinal tract, the remainder being predominantly present in platelets and in the Central Nervous System (CNS). 5-HT is implicated in a vast array of physiological and pathophysiological pathways. In the periphery, it contracts a number of smooth muscles and induces endothelium-dependent vasodilation. In the CNS, it is believed to be involved in a wide range of functions, including the control of appetite, mood, anxiety, hallucinations, sleep, vomiting and pain perception.
Neurons that secrete 5-HT are termed serotonergic. The function of 5-HT is exerted upon its interaction with specific (serotonergic) neurons. Seven types of 5-HT receptors have been identified: 5-HT1 (with subtypes 5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E and 5-HT1F), 5-HT2 (with subtypes 5-HT2A, 5-HT2B and 5-HT2C), 5-HT3, 5-HT4, 5-HT5 (with subtypes 5-HT5A and 5-HT5B), 5-HT6 and 5-HT7. Most of these receptors are coupled to G-proteins that affect the activities of adenylate cyclase or phospholipase Cγ.
Alterations in the activity of multiple neurotransmitter receptor systems (dopamine, serotonin, glutamate, GABA, acetylcholine) have been implicated in the manifestation of the symptoms of schizophrenia. The most widely accepted “Dopamine Hypothesis of Schizophrenia” in its simplest form states that the positive symptoms of this pathology relate to a functional hyperactivity of the mesolimbic dopaminergic system, while the negative and cognitive aspects can be traced to a functional hypoactivity of the mesocortical dopaminergic projections. Atypical antipsychotics block the mesolimbic dopaminergic neurotransmission, thereby controlling positive symptoms, with little or no effect on the nigrostriatal system, leading to less induction of extrapyramidal side effects (EPS).
Primary negative and cognitive symptoms of schizophrenia reflect a dysfunction of the frontal cortex (“hypofrontality”), which is thought to be induced by a decreased tone in the mesocortical dopaminergic projection field [Davis K L, Kahn R S, Ko G and Davidson M (1991). Dopamine in schizophrenia: a review and re-conceptualization. Am J Psychiatry 148: 1474-86. Weinberger D R and Berman K F (1996). Prefrontal function in schizophrenia: confounds and controversies. Philos Trans R Soc Lond B Biol Sci 351: 1495-503]. Agents that selectively enhance dopamine levels in the cortex have the potential to address the negative symptoms of this disorder. Atypical antipsychotics lack robust efficacy against negative and cognitive components of the schizophrenic syndrome.
The schizophrenic symptomatology is further complicated by the occurrence of drug-induced so-called secondary negative symptoms and cognitive impairment, which are difficult to distinguish from primary negative and cognitive symptoms [Remington G and Kapur S (2000). Atypical antipsychotics: are some more atypical than others?Psychopharmacol 148: 3-15]. The occurrence of secondary negative symptoms not only limits therapeutic efficacy but also, together with these side effects, negatively affects patient compliance.
It may thus be hypothesized that a novel mechanistic approach that blocks dopaminergic neurotransmission in the limbic system but does not affect the striatal and pituitary projection fields, and stimulates frontocortical projection fields, would provide an efficacious treatment for all parts of the schizophrenic pathology, including its positive, negative and cognitive symptoms. Moreover, a selective compound that is substantially free of the ancillary pharmacology that characterizes current agents would be expected to avoid a variety of off-target side effects that plague current treatments such as extrapyramidal side effects (EPS) and weight gain.
The 5-HT2C receptor, previously named 5-HT1C, is a G-protein-coupled receptor, which couples to multiple cellular effector systems including the phospholipase C, A and D pathways. It is found primarily in the brain and its distribution is particularly high in the plexus choroideus, where it is assumed to control cerebrospinal fluid production [Kaufman M J, Hirata F (1996) Cyclic GMP inhibits phosphoinositide turnover in choroid plexus: evidence for interactions between second messengers concurrently triggered by 5-HT2C receptors. Neurosci Lett 206:153-156]. Very high levels were also found in the retrosplenial, piriform and entorhinal cortex, anterior olfactory nucleus, lateral septal nucleus, subthalamic nucleus, amygdala, subiculum and ventral part of CA3, lateral habenula, substantia nigra pars compacta, several brainstem nuclei and the whole grey matter of the spinal cord [Pompeiano M, Palacios J M, Mengod G (1994). Distribution of the serotonin 5-HT2 receptor family mRNAs: comparison between 5-HT2A and 5-HT2C receptors. Brain Res Mol Brain Res 23:163-178]. A comparison of the distribution of 5-HT2C mRNA with that of 5-HT2C protein in monkey and human brains has revealed both pre- and postsynaptic localization [Lopez-Gimenez J F, Mengod G, Palacios J M, Vilaro M T (2001) Regional distribution and cellular localization of 5-HT2C receptor mRNA in monkey brain: comparison with [3H]mesulergine binding sites and choline acetyltransferase mRNA. Synapse 42:12-26].
It is anticipated that modulation of the 5-HT2C receptor will improve disorders such as depression, anxiety, schizophrenia, cognitive deficits of schizophrenia, obsessive compulsive disorder, bipolar disorder, migraine, epilepsy, substance abuse, eating disorders, obesity, diabetes, sexual dysfunction/erectile dysfunction, sleep disorders, psoriasis, Parkinson's disease, pain conditions and disorders, and spinal cord injury, smoking cessation, ocular hypertension and Alzheimer's disease. Modulators of the 5-HT2C receptor are also shown to be useful in the modulation of bladder function, including the prevention or treatment of urinary incontinence.
There is an ongoing need for providing compounds having high affinity and selectivity for the 5-HT2C receptor. In particular the compounds should have low affinity to adrenergic receptors, such as the α1-adrenergic receptor, histamine receptors, such as the H1-receptor, and dopaminergic receptors, such as the D2-receptor, in order to avoid or reduce side effects associated with modulation of these receptors, such as postural hypotension, reflex tachycardia, potentiation of the antihypertensive effect of prazosin, terazosin, doxazosin and labetalol or dizziness associated with the blockade of the α1-adrenergic receptor, weight gain, sedation, drowsiness or potentiation of central depressant drugs associated with the blockade of the H1-receptor, or extrapyramidal movement disorder, such as dystonia, parkinsonism, akathisia, tardive dyskinesia or rabbit syndrome, or endocrine effects, such as prolactin elevation (galactorrhea, gynecomastia, mentstrual changes, sexual dysfunction in males), associated with the blockade of the D2-receptor.
The present invention provides compounds which have an affinity for the 5-HT2C, thus allowing the treatment of disorders related to or affected by the 5-HT2C receptor.